Bypass of receptor-mediated resistance to colicin E3 in Escherichia coli K-12

Synergism of Water Shock and a Biocompatible Block Copolymer Potentiates the Antibacterial Activity of Graphene Oxide

Graphene oxide (GO) is promising in the fight against pathogenic bacteria. However, the antibacterial activity of pristine GO is relatively low and concern over human cytotoxicity further limits its potential. This study demonstrates a general approach to address both issues. The developed approach synergistically combines the water shock treatment (i.e., a sudden decrease in environmental salinity) and the use of a biocompatible block copolymer (Pluronic F-127) as a synergist co-agent. Hypoosmotic stress induced by water shock makes gram-negative pathogens more susceptible to GO. Pluronic forms highly stable nanoassemblies with GO (Pluronic-GO) that can populate around bacterial envelopes favoring the interactions between GO and bacteria. The antibacterial activity of GO at a low concentration (50 μg mL(-1) ) increases from <30% to virtually complete killing (>99%) when complemented with water shock and Pluronic (5 mg mL(-1) ) at ≈2-2.5 h of exposure. Results suggest that the enhanced dispersion of GO and the osmotic pressure generated on bacterial envelopes by polymers together potentiate GO. Pluronic also significantly suppresses the toxicity of GO toward human fibroblast cells. Fundamentally, the results highlight the crucial role of physicochemical milieu in the antibacterial activity of GO. The demonstrated strategy has potentials for daily-life bacterial disinfection applications, as hypotonic Pluronic-GO mixture is both safe and effective.

Activation of colicin M by the FkpA prolyl cis-trans isomerase/chaperone

Colicin M (Cma) is specifically imported into the periplasm of Escherichia coli and kills the cells. Killing depends on the periplasmic peptidyl prolyl cis-trans isomerase/chaperone FkpA. To identify the Cma prolyl bonds targeted by FkpA, we replaced the 15 proline residues individually with alanine. Seven mutant proteins were fully active; Cma(P129A), Cma(P176A), and Cma(P260A) displayed 1%, and Cma(P107A) displayed 10% of the wild-type activity. Cma(P107A), Cma(P129A), and Cma(P260A), but not Cma(P176A), killed cells after entering the periplasm via osmotic shock, indicating that the former mutants were translocation-deficient; Cma(P129A) did not bind to the FhuA outer membrane receptor. The crystal structures of Cma and Cma(P176A) were identical, excluding inactivation of the activity domain located far from Pro-176. In a new peptidyl prolyl cis-trans isomerase assay, FkpA isomerized the Cma prolyl bond in peptide Phe-Pro-176 at a high rate, but Lys-Pro-107 and Leu-Pro-260 isomerized at only <10% of that rate. The four mutant proteins secreted into the periplasm via a fused signal sequence were toxic but much less than wild-type Cma. Wild-type and mutant Cma proteins secreted or translocated across the outer membrane by energy-coupled import or unspecific osmotic shock were only active in the presence of FkpA. We propose that Cma unfolds during transfer across the outer or cytoplasmic membrane and refolds to the active form in the periplasm assisted by FkpA. Weak refolding of Cma(P176A) would explain its low activity in all assays. Of the four proline residues identified as being important for Cma activity, Phe-Pro-176 is most likely targeted by FkpA.

Mapping functional domains of colicin M

Colicin M (Cma) lyses Escherichia coli cells by inhibiting murein biosynthesis through hydrolysis of the phosphate ester between C(55)-polyisoprenol and N-acetylmuramyl (MurNAc)-pentapeptide-GlcNAc in the periplasm. To identify Cma functional domains, we isolated 54 point mutants and small deletion mutants and examined their cytotoxicity levels. Activity and uptake mutants were distinguished by osmotic shock, which transfers Cma into the periplasm independent of the specific FhuA receptor and the Ton system. Deletion of the hydrophobic helix α1, which extends from the compact Cma structure, abolished interference with the antibiotic albomycin, which is transported across the outer membrane by the same system as Cma, thereby identifying α1 as the Cma site that binds to FhuA. Deletion of the C-terminal Lys-Arg strongly reduced Cma translocation across the outer membrane after binding to FhuA. Conversion of Asp226 to Glu, Asn, or Ala inactivated Cma. Asp226 is exposed at the Cma surface and is surrounded by Asp225, Asp229, His235, Tyr228, and Arg236; replacement of each with alanine inactivated Cma. We propose that Asp226 directly participates in phosphate ester hydrolysis and that the surrounding residues contribute to the active site. These residues are strongly conserved in Cma-like proteins of other species. Replacement of other conserved residues with alanine inactivated Cma; these mutations probably altered the Cma structure, as particularly apparent for mutants in the unique open β-barrel of Cma, which were isolated in lower yields. Our results identify regions in Cma responsible for uptake and activity and support the concept of a three-domain arrangement of Cma.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Bacteriocin 28b from Serratia marcescens N28b: identification of Escherichia coli surface components involved in bacteriocin binding and translocation

Serratia marcescens N28b produces bacteriocin 28b, active against Escherichia coli. Bacteriocin sensitivity tests performed on a collection of E. coli envelope mutants, and isolation and characterization of E. coli bacteriocin-28b-insensitive mutants, showed that the core lipopolysaccharide, outer membrane proteins OmpA and OmpF, and TolQ, TolA, and TolB proteins are involved in bacteriocin 28b lethal activity. These mutants are assayed for bacteriocin 28b sensitivity under normal and bypass conditions, and their bacteriocin-binding ability was determined. The results obtained suggest that the core lipopolysaccaride and outer membrane proteins OmpA and OmpF are involved in bacteriocin 28b binding. Furthermore, bacteriocin 28b translocation requires proteins TolA, TolB, and TolQ.

Colicins: structures, modes of action, transfer through membranes, and evolution

This article intends to inform a broader audience on a fascinating class of protein toxins (bacteriocins) which usually kill only cells of the same species. Those who gained a deeper interest in bacteriocins can find a comprehensive description of the field in a recent book based on a conference (James et al. 1992), and in more specialized review articles dealing with certain aspects (Pugsley 1984a, b), or certain colicins (De Graaf and Oudega 1986; Harkness and Olschläger 1991; Lazdunski et al. 1988). The older literature has been reviewed by Brandis and Smarda (1971), Reeves (1972), Hardy (1975) and Konisky (1982).

Functional domains of S-type pyocins deduced from chimeric molecules

Functional domain structures of pyocins AP41, S1, and S2 were assigned by examining the functions of chimeric pyocins and deletion derivatives. Pyocins AP41, S1, and S2 are essentially composed of three domains, the receptor-binding domain, the translocation domain, and the DNase domain, in that order from the N terminus to the C terminus. The alignment of these domains is distinct from that in E2-group colicins with functions similar to those of these pyocins. Pyocins AP41 and S2 have a fourth domain between the receptor-binding and the translocation domains, which is dispensable for their killing functions.

Domains of colicin M involved in uptake and activity

Colicin M inhibits murein biosynthesis by interfering with bactoprenyl phosphate carrier regeneration. It belongs to the group B colicins the uptake of which through the outer membrane depends on the TonB, ExbB and ExbD proteins. These colicins contain a sequence, called the TonB box, which has been implicated in transport via TonB. Point mutations were introduced by PCR into the TonB box of the structural gene for colicin M, cma, resulting in derivatives that no longer killed cells. Mutations in the tonB gene suppressed, in an allele-specific manner, some of the cma mutations, suggesting that interaction of colicin M with TonB may be required for colicin M uptake. Among the hydroxylamine-generated colicin M-inactive cma mutants was one which carried cysteine in place of arginine at position 115. This colicin derivative still bound to the FhuA receptor and killed cells when translocated across the outer membrane by osmotic shock treatment. It apparently represents a new type of transport-deficient colicin M. Additional hydroxylamine-generated inactive derivatives of colicin M carried mutations centered on residues 193-197 and 223-252. Since these did not kill osmotically shocked cells the mutations must be located in a region which is important for colicin M activity. It is concluded that the TonB box at the N-terminal end of colicin M must be involved in colicin uptake via TonB across the outer membrane and that the C-terminal portion of the molecule is likely to contain the activity domain.

The tol gene products and the import of macromolecules into Escherichia coli

Genetic studies have identified a number of genes whose products appear to be required for the transport of the group A colicins and the single-stranded DNA of certain filamentous bacteriophages into Escherichia coli. Mutations in these genes allow normal binding of the colicins to their outer-membrane receptors and of the bacteriophage of the tip of specific conjugative pili, but do not allow translocation of the macromolecules to their target. These mutations have been designed 'tolerant' (tol) mutations and the protein products specified by these genes appear to comprise part of a transport system known as the Tol import system. Some of these genes have been isolated, sequenced and their protein products localized to the membranes or periplasm of E. coli. Information is also available regarding the domains of the colicins or phage proteins which interact with the Tol proteins. A preliminary model of the location and possible interactions of the Tol proteins is presented.

Individual domains of colicins confer specificity in colicin uptake, in pore-properties and in immunity requirement

Six different hybrid colicins were constructed by recombining various domains of the two pore-forming colicins A and E1. These hybrid colicins were purified and their properties were studied. All of them were active against sensitive cells, although to varying degrees. From the results, one can conclude that: (1) the binding site of OmpF is located in the N-terminal domain of colicin A; (2) the OmpF, TolB and TolR dependence for translocation is also located in this domain; (3) the TolC dependence for colicin E1 is located in the N-terminal domain of colicin E1; (4) the 183 N-terminal amino acid residues of colicin E1 are sufficient to promote E1AA uptake and thus probably colicin E1 uptake; (5) there is an interaction between the central domain and C-terminal domain of colicin A; (6) the individual functioning of different domains in various hybrids suggests that domain interactions can be reconstituted in hybrids that are fully active, whereas in others that are much less active, non-proper domain interactions may interfere with translocation; (7) there is a specific recognition of the C-terminal domains of colicin A and colicin E1 by their respective immunity proteins.

Resistance of Escherichia coli to osmotically introduced complement component C9

Investigation into the action of osmotically introduced C9 in Escherichia coli (in the absence of any other complement components) revealed that C9 could inhibit inner membrane respiration and cause a decrease in the viability of cells that were normally complement sensitive. This effect is analogous to the loss of inner membrane function and viability due to the assembly of the C5b-9 complex on these cells. Complement-resistant cells showed no such inhibition of respiration or loss of viability when subjected to the osmotic introduction of C9. The reason for this failure of C9 to affect complement-resistant cells was explored to determine whether this resistance to C9 was due to an inability of proteins in general to be osmotically introduced into the complement-resistant cells. The protein toxins melittin and colicin E1 were showed to be able to kill these complement-resistant cells (as well as complement-sensitive cells) when osmotically introduced into the periplasm. Therefore, cellular resistance to osmotically introduced C9 is not due to an inability of proteins to be introduced into the cells and may be related to a mechanism of cellular resistance to the C5b-9 complex.

Involvement of OmpF during reception and translocation steps of colicin N entry

[125I]-colicin N binds to OmpF receptor sites (70,000 per cell) with an average Kassoc of 3.2 x 10(6) M-1 at 23 degrees C. Monoclonal antibody directed against a cell-surface-exposed epitope of OmpF is able to complete with the binding of the colicin in vitro and also to protect against colicin N in vivo. OmpF is an absolute requirement for colicin N uptake. OmpC cannot serve as a substitute for OmpF during translocation across the outer membrane under receptor bypass conditions, which is in contrast to colicin A. Colicin N does not cross-react with various monoclonal antibodies directed against colicin A.

Localization and assembly into the Escherichia coli envelope of a protein required for entry of colicin A

Mutations in tolQ, previously designated fii, render cells tolerant to high concentrations of colicin A. In addition, a short deletion in the amino-terminal region of colicin A (amino acid residues 16 to 29) prevents its lethal action, although this protein can still bind the receptor and forms channels in planar lipid bilayers in vitro. These defects in translocation across the outer membrane in the tolQ cells or the colicin A mutant cannot be bypassed by osmotic shock. The TolQ protein, which is constitutively expressed at a low level, was studied in recombinant plasmid constructs allowing the expression of various TolQ fusion proteins under the control of the inducible caa promoter. The TolQ protein was thus "tagged" with an epitope from the colicin A protein for which a monoclonal antibody is available. A fusion protein containing the entire TolQ protein plus the 30 N-terminal residues of colicin A was shown to complement the tolQ mutation. Pulse-chase labeling followed by gradient fractionation indicated that the bulk of the overproduced fusion protein was rapidly incorporated into the inner membrane, with small amounts localized to regions corresponding to the attachment sites between inner and outer membranes and to the outer membrane itself. However, most of the protein was rapidly degraded, leaving only that localized to the attachment sites and the outer membrane remaining at very late times of chase.

The membrane channel-forming colicin A: synthesis, secretion, structure, action and immunity

The study of colicin release from producing cells has revealed a novel mechanism of secretion. Instead of a built-in 'tag', such as a signal peptide containing information for secretion, the mechanism employs coordinate expression of a small protein which causes an increase in the envelope permeability, resulting in the release of the colicin as well as other proteins. On the other hand, the mechanism of entry of colicins into sensitive cells involves the same three stages of protein translocation that have been demonstrated for various cellular organelles. They first interact with receptors located at the surface of the outer membrane and are then transferred across the cell envelope in a process that requires energy and depends upon accessory proteins (TolA, TolB, TolC, TolQ, TolR) which might play a role similar to that of the secretory apparatus of eukaryotic and prokaryotic cells. At this point, the type of colicin described in this review interacts specifically with the inner membrane to form an ion channel. The pore-forming colicins are isolated as soluble proteins and yet insert spontaneously into lipid bilayers. The three-dimensional structures of some of these colicins should soon become available and site-directed mutagenesis studies have now provided a large number of modified polypeptides. Their use in model systems, particularly those in which the role of transmembrane potential can be tested for polypeptide insertion and ionic channel gating, constitutes a powerful handle with which to improve our understanding of the dynamics of protein insertion into and across membranes and the molecular basis of membrane excitability. In addition, their immunity proteins, which exist only in one state (membrane-inserted) will also contribute to such an understanding.

A hybrid toxin from bacteriophage f1 attachment protein and colicin E3 has altered cell receptor specificity

A hybrid protein was constructed in vitro which consists of the first 372 amino acids of the attachment (gene III) protein of filamentous bacteriophage f1 fused, in frame, to the carboxy-terminal catalytic domain of colicin E3. The hybrid toxin killed cells that had the F-pilus receptor for phage f1 but not F- cells. The activity of the hybrid protein was not dependent upon the presence of the colicin E3 receptor, BtuB protein. The killing activity was colicin E3 specific, since F+ cells expressing the colicin E3 immunity gene were not killed. Entry of the hybrid toxin was also shown to depend on the products of tolA, tolQ, and tolR which are required both for phage f1 infection and for entry of E colicins. TolB protein, which is required for killing by colicin E3, but not for infection by phage f1, was also found to be necessary for the killing activity of the hybrid toxin. The gene III protein-colicin E3 hybrid was released from producing cells into the culture medium, although the colicin E3 lysis protein was not present in those cells. The secretion was shown to depend on the 18-amino-acid-long gene III protein signal sequence. Deletion of amino acids 3 to 18 of the gene III moiety of the hybrid protein resulted in active toxin, which remained inside producing cells unless it was mechanically released.

Bacterial killing by complement. C9-mediated killing in the absence of C5b-8

The ability of serum complement to kill Gram-negative bacteria requires assembly of the membrane attack complex (MAC) on the cell surface. The molecular events that lead to cell killing after MAC assembly are unknown. We have investigated the effect of C9 on bacterial survival in the presence and absence of its receptor, the C5b-8 complex, on the outer membrane. A fluorescence assay of the membrane potential across the inner bacterial membrane revealed that addition of C9 to cells bearing the performed C5b-8 complex caused a rapid and complete dissipation of the membrane potential. No fluorescence change was observed in serum-resistant strains of Escherichia coli. Addition of trypsin, after C9 was bound to C5b-8, did not rescue the cells from the lethal effects of C9. Furthermore, assays of cell killing kinetics and C9 binding indicate that formation of tubular poly(C9) is not required for killing. When C9 was introduced into the periplasmic space in the absence of its receptor by means of an osmotic shock procedure, cell killing occurred. Other proteins, such as C8 or serum albumin, were not toxic, and C9 was ineffective against two resistant strains. The results presented here and previously [Dankert & Esser (1986) Biochemistry 25, 1094-1100], when considered together, indicate that the 'lethal unit' in complement killing of some Gram-negative bacteria is a C9-derived product that acts by dissipation of cellular energy.

A molecular, genetic and immunological approach to the functioning of colicin A, a pore-forming protein

We have constructed, by recombinant DNA techniques, one hybrid protein, colicin A-beta-lactamase (P24), and two modified colicin As, one (P44) lacking a large central domain and the other (PX-345) with a different C-terminal region. The regulation of synthesis, the release into the medium and the properties of these proteins were studied. Only P44 was released into the medium. This suggests that both ends of the colicin A polypeptide chain might be required for colicin release. None of the three proteins was active on sensitive cells in an assay in vivo. However, P44 was able to form voltage-dependent channels in phospholipid planar bilayers. Its lack of activity in vivo is therefore probably caused by the inability to bind to the receptor in the outer membrane. PX-345 is a colicin in which the last 43 amino acids of colicin A have been replaced by 27 amino acids encoded by another reading frame in the same region of the colicin A structural gene; it was totally unable to form pores in planar bilayers at neutral pH but showed a very slight activity at acidic pH. These results confirm that the C-terminal domain of colicin A is involved in pore formation and indicate that at least the 43 C-terminal amino acid residues of this domain play a significant role in pore formation or pore function. Fifteen monoclonal antibodies directed against colicin A have been isolated by using conventional techniques. Five out of the 15 monoclonal antibodies could preferentially recognize wild-type colicin A. In addition, the altered forms of the colicin A polypeptide were used to map the epitopes of ten monoclonal antibodies reacting specifically with colicin A. Some of the antibodies did not bind to colicin A when it was pre-incubated at acidic pH suggesting that colicin A undergoes conformational change at about pH 4. The effects of monoclonal antibodies on activity in vivo of colicin A were investigated. The degree of inhibition observed was related to the location of the epitopes, with monoclonal antibodies reacting with the N terminus giving greater inhibition. The monoclonal antibodies directed against the C-terminal region promoted an apparent activation of colicin activity in vivo.

Localization of the immunity protein-reactive domain in unmodified and chemically modified COOH-terminal peptides of colicin E1

The region of the colicin E1 polypeptide that interacts with immunity protein has been localized to a 168-residue COOH-terminal peptide. This is the length of a proteolytically generated peptide fragment of colicin E1 against which imm+ function can be demonstrated in osmotically shocked cells. The role of particular amino acids of the COOH-terminal peptide in the expression of the immune phenotype was studied. Chemical modification showed that the two histidine residues (His 427 and His 440) and the single cysteine residue (Cys 505) present in the COOH-terminal peptide were not necessary for the colicin-immunity protein interaction. The immunity protein was localized in the cytoplasmic membrane fraction, consistent with previous work of others on the colicin Ia immunity protein and the prediction from the immunity protein amino acid sequence that it is a hydrophobic protein. The distribution of hydrophobic residues along the immunity polypeptide was calculated.

Functional domains of colicin M

The structure of colicin M of Escherichia coli was studied with regard to its organization into functional domains. A proteolytic fragment with an Mr of 24,000 was isolated which comprised the carboxyterminal portion of the protein. It adsorbed to the outer membrane receptor protein and inhibited killing of cells by colicin M and by phage T5 that uses the same receptor. The fragment killed cells when the outer membrane was rendered permeable to macromolecules for a short time by the osmotic shock procedure. It is concluded that the fragment contains the receptor binding site and the active center but is lacking the sequence required for transport into cells. The carboxy-terminal amino acid sequence-Lys-Arg of the fragment was identical to that obtained from colicin M. Release of lysine and arginine led to inactivation of colicin M. The sequence of the first 39 amino acids of the amino terminal end of colicin M was determined.

Murein hydrolase (N-acetyl-muramyl-L-alanine amidase) in human serum

An enzyme was identified in human serum which unlike lysozyme cleaved the amide bond between N-acetyl-muramic acid and L-alanine of the peptide side chain of the rigid layer (murein) of Escherichia coli. The N-acetyl-muramyl-L-alanine amidase released all of the peptide side chains including those to which the lipoprotein is bound. A portion of the peptide side chains of the Micrococcus lysodeikticus murein was also hydrolysed from the polysaccharide chains. E. coli, M. lysodeikticus, Bacillus subtilis and Staphylococcus aureus were not killed by the amidase. Treatment of E. coli with EDTA or osmotic shock rendered the cells sensitive to the amidase and they were killed. Possible biological functions of the amidase are discussed. The enzyme was separated from lysozyme in human serum. Gel permeation chromatography indicated a molecular weight of the active enzyme of 82,000 while gel electrophoresis in the presence of sodium dodecyl sulfate revealed a molecular weight of 75,000. Thus, the enzyme probably consists of a single polypeptide chain. Incubation with neuraminidase rendered the amidase more basic suggesting the release of sialic acid residues. The modified glycoprotein disclosed an increased activity to murein. Enzyme activity was inhibited by p-chloromercuribenzene sulfonate and ethyleneglycol-bis(2-aminomethyl) tetraacetate (EGTA) at 1 and 0.2 mM concentration, respectively, whereas EDTA up to 5 mM was without effect. The amidase was also inactivated by agents that reduce disulfide bridges.

Reconstitution of maltose transport in Escherichia coli: conditions affecting import of maltose-binding protein into the periplasm of calcium-treated cells

The reconstitution of active transport by the Ca2+ -induced import of exogenous binding protein was studied in detail in whole cells of a malE deletion mutant lacking the periplasmic maltose-binding protein. A linear increase in reconstitution efficiency was observed by increasing the Ca2+ - concentration in the reconstitution mixture up to 400 mM. A sharp pH optimum around pH 7.5 was measured for reconstitution. Reconstitution efficiency was highest at 0 degree C and decreased sharply with increasing temperature. The time necessary for optimal reconstitution at 0 degree C and 250 mM Ca2+ was about 1 min. The competence for reconstitution was highest in exponentially growing cultures with cell densities up to 1 X 10(9)/ml and declined when the cells entered the stationary-growth phase. The apparent Km for maltose uptake was the same as that of wild-type cells (1 to 2 microM). Vmax at saturating maltose-binding protein concentration was 125 pmol per min per 7.5 X 10(7) cells (30% of the wild-type activity). The concentration of maltose-binding protein required for half-maximal reconstitution was about 1 mM. The reconstitution procedure appears to be generally applicable. Thus, galactose transport in Escherichia coli could also be reconstituted by its respective binding protein. Maltose transport in E. coli was restored by maltose-binding protein isolated from Salmonella typhimurium. Finally, in S. typhimurium, histidine transport was reconstituted by the addition of shock fluid containing histidine-binding protein to a hisJ deletion mutant lacking histidine-binding protein. The method is fast and general enough to be used as a screening procedure to distinguish between transport mutants in which only the binding protein is affected and those in which additional transport components are affected.

Exchange of colicin receptor capacity between strains of Escherichia coli sensitive or resistant to colicin K-K235

Phage and colicin-resistant mutants were derived from Escherichia coli K-12P678. Two classes of phage T6 and colicin K-resistant mutants (genotype tsx) were isolated. Tsx-2 mutants, which demonstrated mucoid growth and increased sensitivities to many antibiotics, became sensitive to colicin K when pretreated with ethylenediaminetetraacetate (EDTA), whereas Tsx-1 mutants did not. Reassociation of EDTA-released material partially restored resistance to colicin K for Tsx-2 mutants. When EDTA-released material from strain P678 was associated with either class of K-resistant mutant, an increase in colicin K sensitivity resulted. Observations suggest that colicin K can act on its target site once it penetrates the cell surface. In addition, results suggest that functional colicin K receptors can be transferred from sensitive to resistant strains, thus conferring colicin sensitivity.

Reconstitution of maltose transport in malB mutants of Escherichia coli through calcium-induced disruptions of the outer membrane

The barrier function of the Escherichia coli outer membrane against low concentrations of maltose in strains missing the lambda receptor was partially overcome by treating the cells for 3 h with 25 mM Ca2+. Kinetic analysis of maltose-transport revealed a Ca2+-induced shift of the apparent Km of the system from about 100 microM in cells pretreated with Tris to about 15 microM in cells pretreated with Tris plus Ca2+. In contrast to maltose transport in untreated cells, that of Ca2+-treated lamB cells was inhibited by molecules with a high molecular weight, such as amylopectin (molecular weight, 20,000), and anti-maltose-binding protein antibodies. In addition, lysozyme was shown to attack Ca2+-treated cells in contrast to untreated cells. The Ca2+-induced permeability increase of the outer membrane allowed reconstitution of maltose transport in a mutant missing the maltose-binding protein with osmotic shock fluid containing the maltose-binding protein. Even though Ca2+-treatment allowed the entry of large molecules, the release of the periplasmic maltose-binding protein or alkaline phosphatase was negligible.

Mutations which affect the structure and activity of colicin E3

Among 69 ColE3 mutant plasmids selected on the basis of their inability to produce an active colicin, seven (cop-1 to cop-7) were found to bear a mutation affecting the structural gene for colicin. Three of these (cop-1, cop-2 and cop-3) lead to the production of an inactive colicin molecule which has the same molecular weight as wild-type colicin E3 (67,000). These three inactive colicins are still able to interact with the outer membrane receptor. The cop-1 protein retained the ability to inhibit protein synthesis in vitro and therefore seems specifically affected in it ability to penetrate the cell envelope. The cop-2 and cop-3 proteins lost the ability to inhibit protein synthesis in vitro, and activity which is normally associated with the C-terminal part of the colicin molecule. On the basis of this and further evidence, it is suggested that the cop-2 and cop-3 mutations affect the structure of the C-terminal part of the colicin molecule. The other four mutations (cop-4 to cop-7) lead to the production of colicin-related polypeptides of lower molecular weight (29,000 to 45,000) which display a reaction of partial immunological identity with wild-type colicin. These four polypeptides are unable to interact with the cell surface receptor. Three of these mutants are shown to carry a nonsense mutation.

Penetration of colicin M into cells of Escherichia coli

A new class of colicin M-tolerant mutants of Escherichia coli K-12 was isolated. The mutants exhibited an unusually high tolerance in that they were unaffected by colicin titers of 10(6). The tolerance was confined to colicin M. It was mapped at a locus called tolM, which is close to rpsL. The following gene order was determined: aroE, tolM, rpsL, cysG. The tolerance could be caused by a defect in the uptake of colicin M or by a mutation at the site of action. Insensitive tonA and tonB mutants became sensitive to colicin M upon treatment by osmotic shock, whereas the tolM mutants remained insensitive. Trypsin rescue experiments showed that the tonB-dependent uptake of colicin M required energy like the other tonB-related transport processes. When bound to energy-depleted cells, colicin M prevented adsorption of phage T5. The receptor became accessible to the phage when the cells were energized, except in tonB mutants. These data suggest that the function controlled by the tonB gene is required for the translocation of colicin M from its initial binding site at the tonA-coded receptor protein to the target.

Defective transport and other phenotypes of a periplasmic "leaky" mutant of Escherichia coli K-12

A mutant of Escherichia coli K-12 deficient in high-affinity leucine transport and related binding proteins was obtained by selecting for azaleucine resistance after bacteriophage Mu mutagenesis. We determined that the cause was a generalized loss of periplasmic binding proteins and a sharp decrease in the activity of transport systems requiring them. Other transport systems resistant to osmotic shock and present in membrane vesicles, were affected to a lesser degree or not at all. The mutation, designated lky::Mucts, was shown to be a pleiotropic envelope mutation, rendering the mutant sensitive to ionic and nonionic detergents, antibiotics, and ethylenediaminetetraacetic acid: the strain had also acquired tolerance to colicins E1, E2, and E3, while remaining normally sensitive to a variety of bacteriophages. An analysis of the lipopolysaccharide of parent and mutant strains revealed a twofold reduction in the neutral sugar content of the core oligosaccharide of the lky strain, but no change in sensitivities to phages which utilize lipopolysaccharide or outer membrane proteins for absorption. The lky::Mucts locus was mapped by transduction and found to be located near, or in, the tolPAB gene cluster linked to gal. Secondary mutations suppressing the detergent sensitivity of lky arose at a frequency of 10(-7), yielding a variety of new phenotypes. The lky::Mucts mutation did not give rise to obvious alterations in the gross morphology of the cell or in cell division.